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BOSTON—For the one-third of epilepsy patients refractory to medical therapy, might a beam of light offer them relief? That's the prospect arising from research in “optogenetics” in a rat model of focal epilepsy, presented here at the annual meeting of the American Neurological Association in October.

Optogenetics uses light-sensitive ion channels to control the firing of neurons. The genes for the channels, derived from microorganisms, can be introduced into neurons by viral vectors. When the right channel is stimulated by the right wavelength of light, it can cause the neuron to fire, or most relevant to epilepsy, prevent it from doing so.

“Seizures are intermittent, so it would be a great advance to have a strategy to rapidly and reversibly suppress the seizure, without having to shut down whole brain function,” explained lead investigator Laura Mantoan Ritter, MD, of the Institute of Neurology at University College London in the United Kingdom. “We know high-frequency bursts of activity build up in epileptogenic areas before seizure onset, and we know that resecting them gives patients a better outcome.” However, she noted, about half of patients with medically refractory seizure are also not good surgical candidates.

With that in mind, Dr. Mantoan Ritter set out to test the feasibility of optogenetic control of focal epilepsy. She chose to transfect rats with halorhodopsin, a light-sensitive chloride pump that hyperpolarizes membranes, activated by green light. The gene was cloned into a lentiviral vector, which, she said, was chosen for its combination of relatively large payload capability, minimal immune system activation, and preference for uptake in neurons.

“Our objectives were to target halorhodopsin to excitatory neurons using lentiviruses, and see if we were able to modulate epilepsy activity on demand,” she said.

Focal epilepsy was induced in the animals by injection of tetanus toxin into the motor cortex, leading to high frequency EEG activity, tonic posturing, clonus, and, with high doses, even status epilepticus. One group of animals received halorhodopsin via lentivirus injected into the same site, another received lentivirus alone, and a third received halorhodopsin but no tetanus toxin.

The animals had a small cannula inserted into the same site as the injection, through which she passed an optic fiber hooked up to a laser that could deliver green light, and an EEG recording electrode. This allowed her to monitor seizure onset, and deliver light to raise the firing threshold of the neurons to abort the seizure.

There was no effect on EEG activity from light alone, in the absence of halorhodopsin. But when the ion channel was present, illumination led to a significant decrease in the high-frequency activity associated with seizures. The reduction was quantified by measuring the “coastline length,” or the total length of the EEG line over a given time. More activity means a longer line; treatment with light reduced the length in animals with the channel, but not in animals without it. An automated seizure event classifier similarly revealed reduced seizure activity from light treatment.

“We were able to rapidly and reversibly suppress epileptic activity by activating halorhodopsin in vivo, and we think this opens up the prospect of aborting seizures without disrupting whole brain function,” Dr. Mantoan Ritter said.

However, she was quick to add, there are several large hurdles that remain before moving optogenetics to the epilepsy clinic. “We have to consider the safety of lentiviruses,” to make sure there is no mutagenesis or oncogenesis as a result of treatment. There are initial trials of lentivirus ongoing in humans, which may answer these questions. An alternative virus, adeno-associated virus, is attractive for safety reasons, she said, but its payload capacity is too small for the current generation of opsins.

The light delivery system needs to be made smaller and implantable, rather than requiring communication through the skin, she said, and these developments are under way. She added there is still more work to be done to optimize the timing and duration of illumination. “But we hope that eventually we'll be able to develop implantable EEG defibrillators to abort epileptic seizures,” she said.

If it can be developed, an optogenetic device may offer significant advantages over deep brain stimulation, another focally targeted approach currently in development. “With deep brain stimulation [DBS], often we are unsure about which targets are the best for focal epilepsy,” Dr. Mantoan Ritter said, leading to disruption of circuits beyond the seizure focus. “It is a very nonspecific treatment.” Furthermore, the mechanism of DBS in epilepsy — whether it is stimulating inhibitory circuits or inhibiting excitatory ones, or both — is still in debate, and limits rapid research progress.

EXPERT COMMENTARY

“This study was a nice proof of principle that you can use optogenetics to target a specific cell population,” said Kevin Staley, MD, chief of pediatric neurology at Massachusetts General Hospital in Boston. “All the therapies we use now for epilepsy go to every cell, excitatory and inhibitory. The power of optogenetics is to potentially express this channel wherever you want to. There is almost no limit to how specific you can get. But whether optogenetics will have a role in human epilepsy, it is too early to say.”

Dr. Staley noted that if the numerous technical challenges can be met, it may be possible to contemplate optogenetic treatment of generalized, not just focal, epilepsies, if focal points can be identified that trigger or reinforce the generalized seizure.

The question of the clinical utility of optogenetics “is very much open at this time,” said Sydney Cash, MD, PhD, assistant professor of neurology at Harvard Medical School in Boston and a researcher in epilepsy physiology. The risks of lentiviral treatment and permanent transduction of brain cells with a foreign gene are unknown, and will require long-term research to understand.

“The real power of optogenetics in the immediate term is that it is going to allow us to ask questions about how seizures start and stop,” he said. The ability to target very small groups of cells, and to reliably turn them on and off, is likely to lead to important insights into seizure initiation, he said, and to answers that would be challenging to obtain in other ways.

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